THE COLLOIDAL STATE 



95 



of the former, containing particles with a diameter of 90 yu/u,, required a concentration 

 of hydrochloricacid of 1 molecule per litre. Another specimen with particlesof 2 10 fip. 

 required only O5 molar. When the particles were too small to be resolved by the 

 ultra-microscope, 0'3 molar sodium chloride was required, whereas particles of 

 210 fj.[ji only needed 0'07 molar solution of sodium chloride. It will also be noted 

 that the smaller the particles, the greater the changes of surface energy involved 

 in aggregation. 



The fact that precipitation is due to inequality and irregular distribution of 

 electric charges, as in the experiment of Mines related above, explains why the 

 effect of a given amount of electrolyte depends on the suddenness with which it is 

 added, as found by Freundlich (1903, pp. 145 and 151). If a quantity capable of 

 precipitating, when added all at once, be added in small portions at a time, a process 

 of acclimatisation or tolerance (" Gewohnung ") is established and no apparent 

 effect is produced, because the particles have all been equally affected by the 

 electrical changes. 



When the electric charge is due to surface ionisation, the mode of action of an 

 electrolyte may be analysed further in the following way (Freundlich and Elissafov, 

 see Elissafov, 1912, p. 411): The charge is due to the different solution tensions 

 of the ions of the comparatively insoluble matter of which the suspended particles 

 consist. On the surface of such a substance as glass, for example, there is a layer 

 of ionising silicate, tending to go into true solution in the water ; the K 1 and 

 Na* ions have a great solution tension and form an outer layer ; the almost 

 insoluble, slowly diffusing, perhaps strongly adsorbed, silicate ions form an inner 

 layer which, attached to the solid particle, give it the properties of a huge 

 multivalent ion, the colloidal ion of Hardy. The essential diffei'ence between this 

 and an ordinary ion is that, on account of the size of the colloidal ion, surface 

 actions come into play, so that differences in concentration in its neighbourhood 

 are produced by adsorption. Now, according to the law of mass action, there is 

 a constant relation between the product of the concentrations of anion and cation 

 on the one hand, and the concentration of the non dissociated electrolyte on the 

 other hand. Or, as usually expressed : 



(anion) (cation) = K (non-dissociated salt). 



Applied to the multivalent colloidal anion of the case before us : 

 (multivalent anion) (cation) K (non-dissociated salt). 



This implies that the concentration of the cation determines that of the 

 multivalent anion, in other words, the charge on the surface, so that the cation 

 of an electrolyte added will diminish or annul the concentration of the anion of 

 the surface, and with it the electric charge. For further details of this point of 

 view, the reader is referred to the paper quoted. 



It is interesting to note that, according to the experiments of Dumanski (1910), substances 

 which show all the signs of being in true solution can be converted, by the action of neutral 

 salts, into the colloidal state. For example, solutions of molybdenum oxide showed no signs 

 of heterogeneity under the ultra-microscope, not even a diffused illuminated cone ; the 

 depression of the freezing point also showed that the molecules present were not polymerised. 

 On the addition of ammonium or barium chloride, or other salts, a colloidal solution was 

 formed by coalescence of the molecules. 



There is a difficulty sometimes felt with regard to the precipitation of colloids by electro- 

 lytes which must be mentioned, since it is not satisfactorily explained. When one ion of 

 the precipitating salt is carried down with the coagulum, the other ion must be left free. 

 To take a case, it seems that Cl' ion must be left when calcium chloride acts upon arsenious 

 sulphide. Even if we suppose that more water is dissociated to give the increase of H' ion 

 shown by the acid reaction, there still remains the corresponding OH' ion to be accounted for. 



EMULSOIDS 



The class of colloidal solutions of most importance to the physiologist is that 

 variously called emulsoid, lyophile, stable, or reversible. These four names, how- 

 ever, although in general applicable to the majority of members of the class, are 

 not, strictly speaking, synonymous. Owing to the existence of all stages of 

 transition, it is natural to find that certain of these characteristics may be absent 



